Personally I have never used the ‘look at the end of the runway’ technique. And indeed on certain upslope runways you will not see the end of the runway at all during the flare!
Having said that, uphill slopes can be tricky and only through experience I’ve learned to adjust my flare technique ever so slightly to adjust for upslope, downslope, glide slope angle. The first step is realizing that you will have to adjust your flare technique (ever so slightly, nothing drastic!!) to achieve the same result, it’s pure physics. You have opened this topic so, at least you are aware of the issue and others will reinforce this. The rest will come with experience.
(I’m no trainer as you probably can tell, consider this just the patient advice of your regular line captain)